Data communication equipment (DCE) such as modems are generally utilized to transmit and receive data over a communication channel. High speed modems typically use bandwidth-efficient modulation schemes such as quadrature modulation. In such a system that utilizes binary data, the data is first mapped into a sequence of complex signal points or symbols selected from a constellation with a finite number of points. A real-valued transmitted signal is utilized to carry information about this complex sequence.
One category of DCEs, often referred to as full-duplex modems, is capable of transmitting and receiving simultaneously over a two-wire communication link such as a telephone channel in the general switched telephone network. On such a two-wire telephone channel, hybrid couplers are commonly used to perform two to four wire conversions which separate the data transmission in both directions. Due to imperfect impedance matching at the hybrid couplers, data transmission separations are not ideal. Thus, there exist echoes that interfere with normal data transmission. Such echoes may be divided into two categories: a talker echo and a listener echo. The talker echo signal, generated by an outgoing signal from a local modem's transmitter, typically includes a near echo component and a far echo component. The near echo is generated by hybrids in the local modem and a near-end telephone central office, and the far echo signal is mainly generated by hybrids in a remote central telephone office and a remote modem. The far echo signal is delayed in time relative to the near echo signal since it travels a round trip around the telephone channel. The far echo signal may also be corrupted by a frequency offset in the telephone network. The listener echo signal, in contrast, is generated by a data signal from the remote modem's transmitter. The overall channel that the listener echo signal passes through can be viewed as a cascade of the channel that a normal received data signal passes through and an extra channel that includes the round trip telephone trunk and two trans-hybrid paths on both sides of the trunk, as is illustrated in FIG. 1. The listener echo signal is delayed in time with respect to the normal received data signal since it travels an extra round trip around the telephone network. Both the talker far echo delay and the listener echo delay are about equal to the telephone trunk round trip delay, and hence are nearly equal to each other.
Similar to the talker far echo signal corruption, the listener echo signal may also be corrupted by a frequency offset, often designated as phase roll, thereby complicating cancellation of that echo signal. Thus, phase correction circuitry is needed to track and correct phase variations in the echo.
The listener echo total frequency offset consists of a sum of two components. The first component corresponds to the normal received data frequency offset, while the second component corresponds to the frequency offset of the listener echo relative to the normal received data signal, which is substantially equal to the talker far echo frequency offset. Further, the second component is substantially equal to a sum of the normal data frequency offsets in the transmission and the reception directions, typically being smaller than the first component and a total frequency offset.
Most existing two-wire full-duplex high-speed modems come equipped with an adaptive talker echo canceller that is capable of nearly eliminating the talker echoes, including the near and the far talker echoes. The listener echo is often much weaker than the normal received data signal. The ratio between the normal received data signal and the listener echo signal is computed as the total signal loss by the round trip telephone trunk and trans-hybrid losses on both sides of the telephone trunk. When this ratio is much greater than the required signal-to-noise ratio (SNR), the listener echo signal has little effect on the reception of the remote data. However, as the quality of the telephone network improves, the signal loss by the telephone trunk may be reduced, and thus the ratio between the normal received data signal and the listener echo signal may also be reduced. As a result, the listener echo signal may become problem, especially for very high-speed modems which may require a higher SNR to obtain reliable reception. In this case, elimination of the listener echo signal becomes necessary.
In the prior art listener echo cancellers have been implemented as a special form decision feedback equalizer. A detected data signal from an output of a decision device, after an appropriate delay corresponding to a listener echo delay, is fed back to a transversal filter whose coefficients are adaptively adjusted such that the filter's output, after correcting a total phase variation caused by a total frequency offset in the listener echo signal, becomes an estimate of the listener echo signal. This estimate is then typically subtracted from a phase-corrected equalizer output before it is sent to the decision device. Such a structure has two potential problems. First, as is the case for a conventional decision feedback equalizer, decision errors in the decision device may cause error propagation. Second, to obtain a more reliable decision, complex decision algorithms, such as a Viterbi decoding algorithm, are generally employed. A relatively long decision delay may be required for such a decoding algorithm. If the listener echo delay is shorter than this decision delay, the decision will not be readily available for the listener echo canceller. In addition, a major drawback is that a relationship between the listener echo signal and the talker far echo signal is typically disregarded, forcing estimation of the listener echo bulk delay by an extensive search scheme. Prior art compensates the total frequency offset using a simple first-order phase-locked loop (PLL). Thus, as is known in the art, an inherent compensation error by such a first-order PLL is proportional to the frequency offset to be compensated.
Hence, there is a need for a listener echo signal canceller that eliminates the problem of decision error propagation and that is not limited by decision delay, that utilizes information obtained for the talker far echo cancellation, and that improves phase correction.